Various golf club designs have been introduced in the marketplace to address the desire for equipment that has a long service life. In particular, manufacturers have produced club heads from a range of materials including metals and alloys such as stainless steel, aluminum, and titanium. In addition, club heads with variable face thickness have been developed, so that the stresses associated with the impact of a golf ball with a club face may be properly managed.
Titanium alloys typically used for golf club head manufacture include 6-4 (Ti-6% Al-4% V), due to its high strength to density ratio and stiffness (modulus of elasticity), and 15-3-3-3 (Ti-15% V-3% Cr-3% Al-3% Ni). Club faces may also be cold forged or stamped from as-rolled sheet stock of high strength SP-700 titanium alloy (Ti-4.5% Al-3% V-2% Mo-2% Fe). In addition, the face can be formed of a high strength forging titanium alloy such as 10-2-3 (Ti-10% V-2% Fe-3% Al). Typical face thicknesses for titanium alloy metal woods range from about 0.118 to 0.126 inches.
It is desirable to decrease the thickness of the face of the club head in order to redistribute the weight elsewhere within the head. However, there are limitations on the degree of thinness of the club face, as a function of the face material and treatment. For example, a club face that is too thin may be susceptible to catastrophic failure during impact, as it may be unable to withstand the impact stresses. Failure may occur, for example, proximate the impact region, as well as proximate high stress concentration zones such as scorelines. In addition, the fatigue life of a given club head may be decreased as a result of the thin club face, which may lead to premature failure of the club head after repeated stressing. Variable face thickness addresses some of the problems inherent with thin face designs; by providing an area of increased thickness proximate the center of the face, where ball impacts are intended to occur, a stronger club head may be obtained so that the stresses of impact do not affect the integrity of the club head. Adjacent regions may have reduced thickness as compared to the central region of the face, so that weight may be redistributed in the club head.
Mechanical and chemical surface treatments have long been used in golf club manufacturing to produce club heads with high quality, finished, exterior surfaces. The treatments, including shot peening, are typically used for aesthetic conditioning, i.e. creating exterior surfaces with a desirable look and feel, such as by cleaning or selectively roughening or smoothing the surface. The surfaces of some iron golf clubs from Taylor Made, Callaway, and Ping have been finished with shot peening. For example, Taylor Made's ICW-11 irons, produced during the 1980s, were treated with shot peening. In addition, the stainless steel heads of Wilson Power Chamber metal wood golf clubs were shot peened on their outer surface. Although shot peening treatments have become accepted methods of conditioning the exterior of club heads, a drawback to such treatments is that the peening media may penetrate the club head in thin areas that cannot withstand the repeated, focused impacts.
U.S. Pat. No. 5,487,543 to Funk discloses a shot peened golf club head. The exposed ball striking surface of a golf iron club head is subjected to a very high intensity shot peening to develop an increase in hardness and a compressive stress on the surface. The shot peening is used to work harden the club head face and develop a compressive stress on the surface by cold working.
Despite these developments, there exists a need for an improved golf club head with a compressive layer on the inner surface of the club face. More particularly, there is a need for a golf club head with a compressive layer on the inner surface of the club face that is formed by peening the inner surface.